The present invention relates to a mass spectrometer and a method of mass spectrometry.
Many time of flight (TOF) detector instruments employ electron multiplier detectors, such as microchannel plate detectors (MCPs) or discrete or continuous dynode detectors. A common feature of these detectors is that primary ions strike the detector, releasing secondary electrons which are guided to further electron multiplication stages. The conversion efficiency or electron yield from an ion strike to the production of secondary electrons defines the efficiency of the detector. Researchers have previously shown that the yield (λ) with which an ion generates a secondary electron in a MCP is:λ=kmν4.4 where m is the mass of the ion, ν is the velocity of the ion, and k is a proportionality constant with a value of 10−24 and which has a unit that cancels the SI units of mass m and velocity v so as to leave a unitless efficiency λ. The strong velocity dependence of the efficiency λ, means that large mass to charge ratio ions that tend to be relatively slow produce significantly fewer ion counts than faster, smaller mass to charge ratio ions.
In conventional TOF systems where ions of charge q are accelerated through a fixed source potential Vs, the above equation for efficiency λ may be rearranged and approximated as follows:λ˜(4q2Vs2)/m 
For many situations this yield λ is significantly less than unity and researchers have shown that the mechanism for generating signals from high mass ions (>100 kDa) is dominated by the generation of secondary ion yield at the strike surface of the detector. These secondary ions subsequently generate electrons at the next strike surface within the detector. It is therefore apparent that the problem of poor detector efficiency becomes severe when singly charged, high mass to charge ratio ions are analysed. This is a common problem, for example, when analysing large proteins or polymers using matrix assisted laser desorption ionization (MALDI). The detector efficiency may also become a dominant problem for time of flight (TOF) instruments having low acceleration potentials.
In order to maximize the yield of electrons or secondary ions, and hence maximise detector efficiency, many TOF mass spectrometers employ high accelerating voltages so that ions reach the detector with high kinetic energy. In such arrangements, the ions enter the acceleration region at or near ground potential and are then accelerated using high voltages so as to have thousands of electron volts of energy. In order to achieve this the strike surface of the ion detector is held at high potential with respect to the ground potential. In order to allow operation with both positive and negative ions the output of the ion detector is also held at a high voltage. The signals output from the ion detector are generally recorded using time to digital converters (TDCs) or analogue to digital converters (ADCs). However, high speed state of the art TOF system recording electronics operate at or near ground potential and are often sensitive to high voltages. It is therefore a common requirement to isolate the high voltage applied to the output of the ion detector from the ADC or TDC, whilst at the same time allowing the signal arising from the arrival of ions at the detector to be transferred with high fidelity. This may be achieved using capacitive coupling or optical coupling. However, the higher the voltage that is isolated, the more difficult it becomes to provide effective isolation without compromising the fidelity of the ion signal.
In some TOF instruments a post acceleration detector (PAD) is used to increase the detection efficiency for low velocity or low energy ions. In this type of detector ions are accelerated onto a separate conversion dynode and the secondary ions and/or electrons generated therefrom are then accelerated to the strike surface of an electron multiplier. As the secondary charged species formed at the conversion dynode are generally of low mass to charge ratio, their velocity may be significantly higher than the velocity of the primary ion and therefore the efficiency of the detection is increased. However, this approach has the disadvantage that the time response of the detector may be many orders of magnitude slower than in normal operation, which can severely compromise the performance of the mass spectrometer. PAD detectors are therefore commonly used to enhance the efficiency for very high mass to charge ratio species where loss of instrument resolution may be an acceptable compromise. PAD detectors are also employed in mass spectrometers that use low ion acceleration voltages.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.